1. FIELD OF THE INVENTION
This invention relates to a method for chemical milling of metal parts, particularly milling with reduced hydrogen absorption into such parts.
2. THE PRIOR ART
Chemical milling is a useful and cost effective procedure for removing material from metal parts, eg. aircraft and gas turbine engine parts. In the chemical milling process, a metal part, eg. of an aircraft is typically immersed in an acidic milling solution, which solution can be agitated or flowed across the part for metal removal purposes. Such procedure can be used on a wide variety of alloys.
One of the principal disadvantages of the process is the absorption of hydrogen at the surface of the metal part being milled. This is especially true for titanium alloys. In fact some beta titanium alloys, are so sensitive to hydrogen pick-up that multiple baking cycles must be performed to remove any excess hydrogen.
The reason that such parts absorb hydrogen is as follows. The acids used in the chemical milling solution dissolve the material of the metal part at its surface. When the metal surface material dissolves, it takes on a positive charge. Since electrical neutrality of the milling solution must be maintained, some reaction which balances the positive charge of the metals must take place. In an acidic milling solution, the primary reaction is a formation of hydrogen gas (and/or atomic hydrogen) from the hydrogen ions in the acid solution. The reactions described are shown below: EQU M.fwdarw.M.sup.x+ +xe.sup.-
and EQU xH.sup.+ +xe.sup.- .fwdarw.(x/2)H.sub.2 (g)
where M is a metal in the alloy and x is the charge, in solution, of the metal M.
When a metal part is immersed in the milling solution, both of the above reactions occur at the metal part surface. It is the cathodic reaction of hydrogen formation that results in hydrogen pick-up by the material. That is, the metal part absorbs hydrogen with consequent hydrogen embrittlement of such part. Such hydrogen embrittlement seriously weakens the metal part and is unacceptable in, eg. thin-section engine parts which need all the strength such metal can provide.
In the prior art, attempts have been made to solve the above metal part hydrogen absorption problem by adding hydrogen suppressant chemicals to the milling acid bath. See for example, U.S. Pat. No. 4,900,398 to Chen (1990) and U.S. Pat. No. 4,116,755 to Coggins (1978). These references disclose coating the metal part surface with hydrogen suppressant chemicals believed to be, eg. an oxide film which inhibits hydrogen absorption into such metal surface but also can impair the metal removal rate from such part.
Accordingly, there is a need and market for a method for reducing hydrogen pick-up by metal part during a chemical milling process, that does not interfere with the metal removal rate and that otherwise substantially obviates the above prior art shortcomings.
There has now been discovered a chemical milling process that does not require the use of hydrogen suppressant or oxide barrier forming chemicals which prevents or reduces absorption of hydrogen into the metal part during chemical milling.